Deep fat frying has become one of the most popular methods of cooking in domestic, restaurant and industrial establishments throughout the World. Because of the high temperatures involved (typically 160 to 200° C.) it is relatively quick, cooks food right through to the middle, generates a distinctive crust on the food and perhaps most importantly produces rich and complex flavours and food textures, which are very appealing to the consumer.
Frying, whether carried out in oils or fats, however also has a number of well-known disadvantages.
Cooking oil is expensive: high end olive oils are more expensive per liter than petrol or diesel and the price of even lower end cooking oils is comparable to that of petrol or diesel. Cooking oils have to be replaced frequently as the oils degrade during the cooking process, as more fully explained hereinafter. Also cooking oils (and their breakdown products) are absorbed by the food cooked in them which therefore necessitates the operator of a fryer to regularly keep the oil or fat topped up by the addition of extra cooking oil or fat. Cooking in oil therefore comes at a relatively high price compared to boiling in water or roasting in air.
The frequent changing of cooking oil in kitchens, restaurants and industrial sites where food items are manufactured is also a labour intensive and laborious task, which is costly and increases equipment down-time.
Unfortunately it is not possible to extend the life of cooking oils and fats merely by filtering out food debris particles, which frequently accumulate within them. During use cooking oils and fats do not remain unaltered but begin to chemically breakdown. Cooking oils and fats are commonly referred to as triglycerides but are in fact triacylglycerols: i.e. triesters of glycerol (1, 2, 3 propanetriol, which is commonly referred to as glycerine) and three fatty acids. The fatty acids do not need to be of the same type and frequently are not. Common chain lengths for the fatty acids, as determined by gas liquid chromatography, are 12 to 24 carbon atoms with 16 and 18 being particularly favoured. The breakdown of such triglycerides is complex, dependent on numerous factors and is subject to numerous feedback effects but involves three well-understood basic mechanisms: oxidation, polymerisation and hydrolysis.
Oxidation
Oxidation occurs when air comes in contact with frying oil, (see for example Josephson and Lindsey 1987, Journal of Food Sciences, 52, 328 and Fischer and Muller 1991, Potato Research, 34, 159). Oxygen from the air reacts with the two unsaturated carbons at the double-bond via a free radical initiated reaction. The oxidation reaction is promoted by high cooking temperatures (typically 190° C. and above), the presence of metals (including in particular copper and iron) and the presentation to the air of a large surface area of the oil as well as exposure to UV light, which promotes free radical formation. Initially hydroperoxides are produced but these are unstable and at frying temperatures they rapidly break down (via e.g. fission, dehydration and free radical formation) to produce a wide array of secondary oxidation products including polymers, acids, alcohols, esters, aldehydes, methyl ketones, lactones, alcanes, aromatics and other hydrocarbons, (see Belitz and Grosch 1999, Food Chemistry, 2nd edition, Springer-Verlag, Berlin, p. 211).
Some of these secondary oxidation products are volatile and give rise to both pleasant rich flavours but some are also associated with rancid and offensive flavours. For example only 0.08 ppm of pentane is sufficient to reliably produce rancidity, (Warner et al. (1974) Journal of Food Science, 39, 761). Non-volatile compounds, such as core aldehydes, remain in the oil and are absorbed by the food.
Polymerisation
When cooking oil breaks down, the resulting products form both volatile low boiling point and higher boiling point non-volatile compounds. The non-volatile higher boiling point compounds remain within the frying oil and readily polymerize at frying temperatures above 190° C. or in isolated hot spots within the fryer. Such polymerisation products can then bond together to form larger clusters, which can accumulate as an insoluble layer on the surface of the oil, thus preventing water vapour, evaporating from food cooking in the oil, escaping from the oil's surface and thereby producing dangerous foaming, which can lead to fires and personal injury of kitchen staff.
The presence of the impermeable polymer layer in turn promotes more hydrolysis in what can become a runway feed-back driven process. Polymerisation also leads to an increase in the viscosity of the oil which reduces its ability to effect heat transfer and promotes yet more polymerisation. The increase in viscosity also increases the amount of energy required to effect cooking and thus increases energy bills.
Hydrolysis
Hydrolysis is caused by the reaction of water (a weak nucleophile) with the ester linkage in the triacylglycerol molecule to produce initially a diaglyceride and a free fatty acid, which then further breakdown to produce various compounds including lactones or simply boil off, depending on chain length, saturation and other factors. The presence of free fatty acids is frequently associated with a characteristic rancid or acidic flavour.
The production of free fatty acids in cooking oils is additionally problematical for several reasons.
Firstly free fatty acids are one of the main constituents of smoke haze and are both a fire and a health hazard. The smoke point of an oil is the temperature, at which it is seen to start smoking under specified test conditions. The flashpoint of an oil is the temperature at which volatile products are produced in sufficient concentration and quantity to allow ignition. The fire point of an oil is the temperature at which the rate of production of volatile products is sufficiently high to support continuous combustion of the gases emerging from the surface of the oil.
High levels of free fatty acid in cooking oils are associated with reduced smoke, flash and fire points and are thus a significant fire hazard. For example Weiss (Food Oils and Their Uses, Wesport, The AVI Publishing Co. 1983) found that a free fatty acid composition of 0.04% was associated with a smoke point of 218° C., a flashpoint of 327° C. and a fire point of 366° C. whereas for the same oil increasing the free fatty acid content to just 1% percent lead to the smoke point decreasing to 160° C., the flashpoint decreasing to 307° C. and the fire point dropping to 360° C.
In addition to being a fire hazard, an increase in the concentration of free fatty acids (and their break down products) in cooking oils also has deleterious effects on the preparation of food cooked in such oils.
Fatty acids and some of their breakdown products, having both distinct hydrophobic and hydrophilic regions, act as effective surfactants. The effect of the concentration of surfactants in cooking oil on the properties of the food cooked in such oil is well-known (see e.g. Blumenthal MM A New Look At The Chemistry And Physics Of Deep Fat Frying: Food Technology, 1991, 45:2, 68-71, 94). When for example chips are cooked in fresh unused cooking oil they are light in colour and do not have the rich complex aromas associated with fried potatoes. The oil, during this “break in” phase has only low levels of surfactants (such as free fatty acids), which means that the oil has a relatively high surface tension which prevents the oil having close contact with the food. The heat from the oil is not effectively transferred across the oil/wet-food barrier and the food is in part boiled rather than fried as the steam emerging from the food pushes a substantial amount of the oil away from its surface. As the oil is used further the amount of free fatty acid and other surfactants increases resulting in improved food quality. During the so-called optimum phase chips cooked in the oil are golden brown in colour and have a significant crust but with relatively low levels of oil being absorbed by the food, which is cooked through to the centre. For example fresh French fries will typically consist of about 10% by weight of oil, when cooking during the so called optimum phase. However as the oil is subject to both further hydrolysis and oxidation, the increase in free fatty acids and other surfactants decreases the surface tension significantly and ensures that the oil can rapidly bridge the otherwise immiscible oil food barrier. This results in the surface of, for example, chips having a characteristic dark and spotted appearance. Excessive contact with the oil rapidly dries the surface of the food thus trapping moisture in the food and inhibiting heat penetration deeper within the food's centre, which therefore typically is undercooked. The resulting greasy chip with an oil content by weight of typically in excess of about 20%, with a dark spotted exterior and undercooked centre, is familiar to many who have eaten at down market fast food establishments, which do not change their cooking oil often enough.
The absorption of excessive amounts of cooking oils by food cooked in the oil also very significantly increases the calorific value of the food, thus giving many consumers extra calories they do not need and promoting obesity and the numerous health problems associated with it including in particular type II diabetes.
Further the absorption of excessive amounts of cooking oil by food has other important consequences for health. Hydrogenated vegetable oils and fats are widely used in cooking due mainly to their increased stability, resistance to oxidation, longer shelf-life and their greatly increased resistance to rancidity.
However such oils contain increased amounts of trans-fatty acid side chains on the glycerol backbone, which are a material health hazard. After ingestion most of the initial digestion of cooking oils is accomplished in the stomach via specialist pancreatic enzymes (lipases) and bile secretions. The resultant fatty acids and glycerol are then absorbed by cells lining the intestines called enterocytes, where they are re-esterified into triglycerides and transported to the liver as chylomicrons. When the chylomicrons reach the liver, the fatty acids are repackaged into triacylglycerols and phosphatidylcholine and thence into lipoproteins.
High levels of trans fatty acids in the diet are associated with raised serum levels of low density lipoprotein (LDL) cholesterol and with lower levels of high density lipoprotein (HDL) cholesterol in humans. Raised serum LDL and reduced serum HDL levels are associated with coronary artery disease, increased risk of stroke and elevated blood pressure as they decrease the health of the endothelium, the cells lining the arteries of the body which are essential for good cardiovascular health. Studies in humans further demonstrate that trans fats increase inflammation in the body, a potent risk factor for cardiovascular disease, diabetes, and other diseases. Studies in primates have demonstrated that trans fats cause weight gain, especially increasing abdominal fat, which has the greatest metabolic consequences, and is associated with insulin resistance, a known precursor to type II diabetes.
For all these reasons the amount of trans-fatty acids absorbed in the diet should be kept at low levels. One way of achieving that is to reduce the amount of hydrogenated cooking oil absorbed by fried food.
Various ways have been suggested to prolong the useful life of cooking oils. Some of these involve the step of removing the cooking oil from the fryer, followed by the step of subjecting it to one or more treatment methods to remove the contaminants before finally returning the treated oil back to the fryer. Other methods provide for at least the complete cessation of the cooking process, treatment and then the recommencement of the use of the oil.
Oil Removal and Treatment Methods
U.S. Pat. No. 4,112,129 (Duensing et al., Johns Manville) discloses a method of filtering the cooking oil through a composition comprising by weight (i) 47 to 59 parts diatomite, (ii) 28 to 36 parts synthetic calcium silicate hydrate and (iii) 12 to 24 parts synthetic magnesium silicate hydrate.
U.S. Pat. No. 4,681,768A (Mulflur W Jospeph et al) discloses a method for the continuous treatment of cooking oil with a filter made from synthetic calcium silicate. The method involves removal of the oil from the fryer, passing it through the filter and then passing it back into the fryer.
GB 2006729 (Johns Manville) discloses a method for filtering used cooking oils to remove free fatty acids, which uses synthetic calcium silicate but does not disclose an in situ solution suitable for unadapted fryers.
U.S. Pat. No. 5,870,945 discloses a filter cartridge for fitting to a fryer, which includes a mesh housing for containing filtering material which is used to treat the cooking oil outside the fryer prior to its return to the fryer.
U.S. Pat. No. 4,112,129A discloses a method for extending the life of cooking oil by removing free fatty acids which involves treating the oil with a composition of synthetic calcium silicate hydrate and synthetic magnesium silicate hydrate. U.S. Pat. No. 4,112,129A states that the method can be used with conventional cooking oil treatment systems but does not disclose an in situ solution suitable for unadapted fryers which do not have a treatment system.
EP 0226413A discloses a filter container provided with a removable filter bag but which cannot be used during the cooking operation.
U.S. Pat. No. 6,210,732 discloses a method of extending the life of cooking oil by the use of a blend of finely milled citric acid and calcium silicate powder, which is added to the hot oil, left for a certain length of time and then removed by treatment. The U.S. Pat. No. 6,210,732 invention cannot be used during the cooking process.
WO 91/11914A discloses a still further treatment method for used cooking oils, which uses an amorphous silica and alumina composition, which is either added to the hot oil and then filtered out or put in a container which is permeable to the oil but not the treatment composition. The invention disclosed cannot be used during the cooking operation.
U.S. Pat. No. 4,330,564A discloses a method of treating used cooking oil with a composition including a porous carrier, water and a food compatible acid, with the resultant residue being removed by treatment. The invention disclosed cannot be used during the cooking operation.
U.S. Pat. No. 3,947,602A discloses a method of treating cooking oil with a food compatible acid and a suitable adsorbent such as activated carbon. The invention disclosed cannot be used during the cooking operation.
U.S. Pat. No. 5,391,385A discloses the treatment of cooking oil with a mixture of 60-80% amorphous silica and 20 to 40% alumina, the mixture being placed in a permeable container which is then placed in the oil, the container being permeable to the oil but not to the mixture so that the adsorbent is not released into the oil and no treatment is required.
All the above treatment methods either require removal of the oil from the fryer and its treatment before reuse and/or cannot be carried out during the normal frying operation with standard frying equipment, which does not include in-line treatment equipment and a pump.
In Situ Treatment of Cooking Oil
Other methods are known for the treatment of cooking oil in the vessel where cooking takes place.
U.S. Pat. No. 4,764,384A discloses a method of treating used cooking oil with filtering media comprising synthetic amorphous silica, synthetic amorphous magnesium silicate and diatomaceous earth.
U.S. Pat. No. 5,354,570A discloses a method of frying food in cooking oils with a porous rhyolitic powder which selectively reduces the concentration of certain surfactants, whilst the cooking process is on-going.
JP 07-148073A discloses a method of treating cooking oil using finely pulverized zeolite stones which are inserted into a permeable bag which is itself placed into the fryer, with or without food also being present.
The above methods either require the addition of powders to the oil, which is undesirable as they may contaminate and change the texture and taste of any food cooked therein or require a further container to be added to the oil, which will often be problematical during use of the fryer due to space and other constraints.
The WO 2008/015481 and WO 2009/019512 Inventions
WO 2008/015481 and WO 2009/019512 (“the BBM Patents”) (BBM Technology Limited) disclose the use of cementious hydraulically set filters made from ordinary Portland cement (OPC), white cement clinker and mixtures thereof, in the form of standalone briquettes, blocks, pellets, granules or balls, which do not substantially leach calcium or magnesium into cooking oils.
The BBM Patents disclose the use of such treatment elements in cooking oils (a) in situ actually in the frying chamber where the food is being fried during the frying operation and also (b) prior to first use when the cooking oil is in a storage container. WO 2009/019512 additionally discloses the use of film or sheet packaging that resists the ingress of water or water vapour for wrapping the filters, after they have been dried to remove free water after hydraulic setting.
Industrial Frying
Frying is a popular way of treating food products in the food industry. The manufacture of crisps, other fried potato products and pre-fried potato products (particularly chips) is carried on on a substantial scale in many countries. Pre-fried potato products are products that are pre-fried during manufacture, packaged and then finished before consumption, typically in a fast food restaurant, pub or by the end user. Finishing can be carried out by finish or flash frying but oven, air and microwave frying have become more popular over recent years, due to concerns about the health risks associated with cooking in oil.
It is not only potato products that are fried on an industrial scale. Maize products are also processed into a wide variety of snack products, one of them being the Tortilla chip, which accounts for a large percentage of the capacity delivered by the snacks industry.
Fried products produced by the food industry are typically made on a substantial scale, packaged for transport and typically consumed away from the site where they are made. Frying equipment used in the food industry is often substantial is size, containing tens, hundreds or even thousands of liters of cooking oil. For example, equipment used to prepare pre-fried potato chips (French fries) or crisps may have a frying chamber with 1000 liters of oil and possibly up to 5000 or even 10,000 liters of oil. Modern French fry lines typically have a high capacity of between 15 to 30 tonnes per hour, being fed with 30 to 60 tonnes of potatoes an hour.
Such equipment is expensive to buy, often costing tens or even hundreds of thousands of pounds. Operators of such equipment are also reluctant to have the equipment non-operational for any length of time for economic reasons.
It has been found that the filters described in the BBM Patents do not work very effectively if merely put into the frying chambers of such large scale fryers: no material extension of the life of the cooking oil was found using such filters in large industrial fryers when they were simply put into the frying chamber. Without wishing to be bound by any particular theory, it is thought that this was due to the limited circulation of oil due to convection currents in such large frying chambers, Oil located distant from the filters is not able to come into intimate contact with the filter material and thus retains the contaminants and oxidation, hydrolysis and polymerisation breakdown products associated with the degradation of cooking oil during the cooking process.
Further in very large frying equipment it is either impossible due to space constraints or cumbersome and unacceptable to the operator to put large numbers of filters into the fryer. This is particularly true of more modern continuous process fryers, which use much less oil than older models in particular by virtue of having shallower frying chambers, which do not readily accommodate filter briquettes or filter materials in other forms (pellets, balls etc).
There remains therefore a need for a practical way of utilising the treatment elements (made from those generally cementious materials and in accordance with the teaching as both are disclosed in the BBM Patents) in a manner that is suitable for use in larger scale industrial frying equipment.